1. Field of the Invention
This invention relates to methods for treating tubers such as potatoes with sprout inhibitors to prevent sprouting during storage of said tubers.
2. State of the Art
It is well known in the art to treat tubers such as potatoes with various chemicals having sprout-inhibiting properties. CIPC (isopropyl-3-chlorophenyl-carbamate) has been conventionally used for this purpose for about 30 plus years. More recently, chemicals such as various isomers of dimethylnaphthalene and other substituted naphthalenes have exhibited for their sprout-inhibiting characteristics.
Considerable study and innovation have been directed towards inhibition of potato sprouting during storage in large storage facilities. U.S. Patents to Hitchcock et al., U.S. Pat. No. 2,341,868; Luck, U.S. Pat. No. 4,078,480; Vaughn, et al., U.S. Pat. No. 5,129,951; Morgan, U.S. Pat. No. 4,887,525; Plant, U.S. Pat. No. 3,128,170; and Riggle et al., U.S. Pat. No. 5,622,912 describe various treatments to prevent sprouting in storage facilities.
Potatoes when being dug are frequently bruised, cut and/or abraded. These injuries to the potatoes oftentimes cause spoilage during shipment, storage and the like. A process known as suberization occurs naturally which tends to heal many of these injuries. However, whenever potatoes are stored, which occurs with a particularly large portion of potatoes harvested in any given year, if healing occurs slowly, a significant loss of potatoes can occur through spoilage. Early treatment with certain sprout inhibitors, such as CIPC, may retard the suberization process, thus contributing to the loss of potatoes through spoilage.
For example, it is relatively common in the potato storage industry to treat potatoes with Chloroisopropyl-N-carbamate (CIPC) to prevent or retard development of sprouts in the potatoes. Even though untreated potatoes are stored at a cool temperature, for example, generally between about 40.degree. and 45.degree. F., sprouting does begin to occur after a month or more of storage. Storage of upwards of six to eight months is typical for a stored potato harvest. Thus, without treatment of a chemical such as CIPC, the stored potatoes become entangled in sprouts and the whole stored lot of potatoes may become economically useless. Although early treatment with CIPC could be advantageous for sprout inhibition purposes, application of CIPC is typically delayed until after suberization has occurred inasmuch as CIPC tends to retard suberization, resulting in accelerated rot and spoilage.
In potatoes, cell division and cell elongation of the tuber buds results in formation and emanation of sprouts from the tuber buds after the potato has entered a quiescent phase of dormancy that typically follows storage at or slightly above 45.degree. F. Although tuber sprout formation can be suppressed by storage of the tubers at lower temperatures of from 38.degree. to 39.degree. F., the lower storage temperatures cause increased reducing sugar levels in the stored potatoes. Potatoes with increased levels of reducing sugars may turn brown when french fried, thereby producing an unacceptable food product.
To inhibit sprout formation in potatoes, synthetically derived sprout inhibitors, for example, tetrachloronitrobenzene, maleic hydrazide, and isopropyl-3-chlorophenylcarbamate (CIPC) also commonly referred to as chlorpropham, have been applied. CIPC is typically applied in one or two applications to the tubers to be stored using thermal fogging techniques. Conventional thermal fogging involving the application of CIPC into a stream of hot air or onto a hot surface of up to 1000.degree. F., to produce a CIPC aerosol. The CIPC aerosol is circulated through potatoes piled in a potato storage building with the use of fans. Preferably the potatoes are firm rather than soft when treated with the CIPC aerosol, since a pile of softened potatoes may be substantially compressed thereby impeding distribution of the aerosol. CIPC residue levels, will, however, typically decrease over time due to biodegration, venting and atmospheric loss. To extend he effective sprout inhibiting capability of CIPC, further applications may be needed.
However, it is becoming increasingly desirable world-wide to decrease the application of synthetically derived substances to fruits and vegetables during growth, storage and shipping. In particular, residue levels of CIPC are subject to regulation. So, while CIPC has been utilized to inhibit sprout formation in tubers for decades, its toxicology has been questioned and it is one of a number of synthetically derived substances whose residue levels are of concern to the U.S. Environmental Protection Agency.
In order to decrease use of synthetically derived substances such as CIPC, naturally occurring biological control mechanisms and substances are actively sought. Naturally occurring sprout inhibitors are known. For example, U.S. Pat. No. 5,436,226 for NATURAL SUPPRESSION OF SPROUTING IN STORED POTATOES USING JASMONATES claims a method of inhibiting sprouting of tubers by exposure to various forms of jasmonic acid, at some of which are naturally occurring compounds.
Also by way of example, Canadian Patent No. 1,203,394 teaches the use of dimethylnaphthalene (DMN) and diisopropylnaphthalene (DIPN) as potato sprout inhibitors. However, this patent teaches the need for application of DMN and DIPN with an inert carrier which implies the utility of DMN and DIPN alone as the active ingredient. However, long term effectivity of DMN and DIPN as tuber sprout inhibitors at lower residue levels under less than ideal circumstances has not been fully established.
Testing of isomers of dimethyl naphthalene (DMN) as a potential sprout inhibitor was conducted by placing alumina particles containing DMN in a box of potatoes stored at 10.+-.0.5.degree. C. in a ventilated cooler for a period of 12 weeks. The potatoes evidenced sprout inhibition in comparison with a control batch, i.e., in an untreated batch, and with a batch treated with Tecnazene, a commercial sprout inhibitor. Beveridge, et al., The Assessment of Some Volatile Organic Compounds as Sprout Suppressants for Ware and Seed Potatoes, Potato Res. 24 (1981) 61-76 and Beveridge, et al., Dimethyl Naphthalene as a Sprout Suppressant for Ware and See Potatoes, Potato Res., 24 (1981) 77-88.
The use of DMN treatment in a large storage facility has also been proposed. In PCT application PCT/GB92/01482 (Int'l. Pub. No. WO 93102563) of Everett-Todd, a system for monitoring the vapor pressure of alkyl naphthalenes is disclosed wherein alkyl naphthalenes are added to the facility when the monitored vapor pressure of alkyl naphthalenes falls below a certain value, e.g., 5 mg of alkyl naphthalene per cubic meter of storage free space.
While considerable effort has been devoted to sprout prevention of potatoes stored in larger facilities, little or no effort has been devoted to prevention of sprouting of potatoes during shipment. Freshly-dug and stored potatoes are shipped to commercial markets, e.g., restaurants and the like, and to consumer markets, e.g., retail grocery stores, in refrigerated trailers and railroad cars. No other particular effort has been made to inhibit sprouting during the shipping and distribution process.
Riggle et al, supra., teach that simultaneous application of CIPC and DMN provides improved results over application of either sprout inhibitors separately.
Serial application of different sprout inhibitors have previously been conducted where the first sprout inhibitor, maleic hydrazide (MH 30) is applied to potato plants in the field well before the potatoes are harvested. Later, CIPC is applied via an aerosol to the harvested potatoes after they have been harvested and stored for a sufficient period that bruises and cuts have healed, i.e., suberization has occurred.